Dopamine (DA) replacement in Parkinson's Disease (PD) via levodopa therapy lacks long term efficacy and results in debilitating side effects. These obstacles have led researchers to focus on preserving and augmenting endogenous dopaminergic transmission from remaining substantia nigra pars compacta (SNpc) DA neurons. The most promising approach to date is the use of glial cell line-derived neurotrophic factor (GDNF). Preclinical research utilizing GDNF protein infusions and gene transfer clearly demonstrates the ability of GDNF to provide neuroprotection. However, recent results from the controlled clinical trial with GDNF infusions were disappointing, suggesting the need for alternative neuroprotection approaches for PD. Novel therapies that activate signaling pathways implicated in the positive effects of GDNF similarly hold great promise for the treatment of PD. The present predoctoral NRSA proposal will examine the impact of one such molecule, inosine. Inosine has neuroprotective properties in non-parkinsonian models and readily transverses the blood brain barrier. Furthermore, similar to GDNF, inosine has been shown to activate the phosphoinositol-3 kinase (PI3K) and extracellular signal-regulated kinase (ERK) 1/2 signaling pathways. We propose to test the potential of inosine to protect and augment the function of the nigrostriatal pathway in a rodent model of PD. In Aim 1 the effects of inosine on the survival of mesencephalic DA neurons in vitro will be examined. Additional experiments will explore the potential mechanisms of the neuroprotective effects of inosine. In Aim 2 the neuroprotective properties of inosine will be evaluated in vivo, rats will receive daily injections of inosine or saline for a period of 7 weeks beginning 1 week prior to unilateral 6- OHDA infusion into the striatum. Outcome measures will include stereological analysis of SNpc THir neurons, density of striatal THir neurites, intensity of GAP-43 immunoreactivity and extracellular levels of striatal DA and its metabolites via microdialysis. Behavioral evaluations of amphetamine-induced rotational asymmetry and forelimb akinesia also will be performed. Our overall hypothesis is that peripheral administration of inosine will protect SNpc DA neurons and striatal dopaminergic terminals from parkinsonian insult. These studies will determine whether inosine administration can be used as a therapeutic strategy to treat PD. The use of inosine as a therapy for PD holds great promise for patients in all stages of the disease. Many current experimental treatments for PD involve surgical intervention. Inosine's ability to readily enter the brain presents a less invasive alternative to these therapeutic interventions.